April 4, 2013

Last Global ‘Hot Spell’ May Help Improve Current Climate Models

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redOrbit Staff & Wire Reports - Your Universe Online

Temperature patterns during Earth´s last prolonged global “hot spell” some 5.3 million to 2.6 million years ago differed markedly from those of modern times, suggesting current climate models may need to be adjusted to improve future predictions, according to a study published this week in the journal Nature.

The Earth was warmer during that time - known as the Pliocene Epoch - than it is today, and had higher carbon dioxide levels than existed before the Industrial Revolution. However, the waters in the Tropics — off the coast of Peru, for example — were much warmer than they are now, resembling modern El NiÃ±o conditions. There was also little to no east-to-west temperature variation along the equator, and temperature differences between high latitudes and the Tropics were much smaller.

The researchers wanted to better understand this period, which is widely viewed as a potential analog for a future hot Earth, in hopes of improving future climate predictions.

"If we want to understand our future climate, we have to be able to understand the climate of the past," said study co-author Alexey Fedorov, a Yale University climate scientist.

"The Pliocene Epoch attracts particular attention because of similar carbon dioxide levels to what we have had over the last few decades, but its climate was markedly different in several important ways," he said.

"If we're able to simulate early Pliocene climate, however, we will be more confident in our ability to predict future climate change."

It is difficult to model the precise conditions behind the pattern of warming in the early Pliocene, since none of the proposed mechanisms — from high carbon dioxide levels to changes in global ocean circulation patterns — can explain why the ancient warm period looks the way it does, the researchers said.

Study co-author Petra Dekens, Assistant Professor of Geosciences at San Francisco State University, and colleagues wondered whether climate models for the early Pliocene might be missing key processes that could be applied to models of modern climate to improve future climate predictions.

"It's very hard to look at a climate record from the past and say this directly applies to modern climate," Dekens said.

"But what it does do is help us think about what the gaps might be in our models, what are the uncertainties in our current models, and whether those uncertainties could be important."

The Earth´s climate began to change after the early Pliocene, when a pool of warm water spreading out from the equator began to shrink toward lower latitudes, and east-west differences in sea surface temperature began to develop. Overall, the planet's climate shifted toward cooler temperatures.

The researchers were able to see this broad shift in climate by examining a wealth of published data on sea surface temperatures.

Fedorov and colleagues compiled records of sea surface temperatures going back to the early Pliocene five million years ago, which revealed fairly uniform warm temperatures in the Tropics before four million years ago — a scenario that typical climate model simulations fail to show.

These records showed that the early Pliocene climate was "structurally different" from today's climate, Dekens said.

"It's not just that the absolute temperature in any one location is different, it's that the patterns are different."

Dekens' colleagues constructed several models to try and recreate the sea surface temperature conditions of the early Pliocene, but none of the expected "drivers" of climate that they tested could account for all the major features of the ancient climate.

For instance, increases in greenhouse gases and changes in ocean circulation could not reproduce the early Pliocene climate in their models.

Other adjustments to the models, such as reducing the reflection of sunlight by tropical clouds, did bring the models closer to matching the early Pliocene, but still fell short of explaining the full pattern.

Previous attempts to explain Pliocene climate have emphasized tectonic changes in Indonesia and Central America, but accounting for this in climate models still results in a conflict with actual temperature patterns.

The scientists proposed several factors to explain warm temperatures during the Pliocene, including ocean mixing in subtropical waters, perhaps due to widespread hurricanes and diminished cloud reflectivity. When combined in models with higher levels of carbon dioxide, they help replicate conditions of the warm Pliocene Earth.

However, these factors have not been included in climate models used to make future projections, the researchers said.

A better understanding of what drove Pliocene climate, with its nearly uniform tropical ocean temperatures, will increase our confidence in the models, said Fedorov.

"We can't discount a possible future that has a vast pool of warm water covering the tropics, and the changes in atmospheric circulation and rainfall that would go along with that," said study co-author Chris Brierley of the University College London.

"An important question is how much the evidence of climate evolution over the last five million years shapes our assessment of future change. From these observations, it is clear that the climate system is capable of remarkable transformations even with small changes in external parameters such as carbon dioxide," he said.

"Therefore, explaining the discrepancy between model simulations and the early Pliocene temperature patterns is essential for building confidence in our climate projections.

"In many ways, this work on past climates is part of understanding the uncertainty of future climate. It can give us a heads-up of potential climates that we hadn't imagined possible before."